Effect of Cold Gas Inhalation on Cardiac Rate in Man at Depth.

Abstract : Two Navy divers breathed first warm and then cold helium-oxygen mixtures while performing graded exercise on a bicycle ergometer at simulated depths of o, 200, 400, 600, 800, 850, and 1000 feet of seawater. In all cases, heart rate increased in proportion to the increase in oxygen consumption with exercise. When compared with warm gas control values, no consistent changes in heart rate were apparent in either subject during cold gas inhalation through a depth of 800 feet. At 850 and 1000 feet, however, both subjects demonstrated a significant reduction in exercising heart rate on cold gas. The potential mechanisms underlying these changes in cardiac rate and their impact in terms of cardiovascular performance and exercise tolerance are discussed. (Author)